MAGNETISM AND ELECTRICITY 



191 



FIG. 159. A Homemade Dipping 

 Needle. 



The Dipping Needle. Just as the iron filings arranged 

 themselves so that they were lying in the direction of the 

 lines of magnetic force at that point, so a compass needle 

 which is balanced and free to move in a vertical plane 

 will arrange itself parallel 

 with the earth's magnetic 

 lines of force at that place 

 (Figure 158). 



Experiment 53 . Pass two 

 unmagnetized knitting needles 

 through a cork near the center 

 and at right angles to each 

 other (Figure 159). With a pin 

 (a) the system may be adjusted 

 so that it will be perfectly bal- 

 anced. Now magnetize the neeble b, being careful not to change 

 its position in the cork. Replace the needle on the supports so that 

 the N pole will point north. Does the needle balance as before? 



. QUESTIONS 



1. What is the difference between permanent magnetism and 

 induced magnetism? 



2. Name four ways by which the magnetism of a magnet may 

 be destroyed. 



3. What is meant by the magnetic field? 



4. How would an explorer know when he had reached the 

 north or south magnetic pole? 



5. How should magnets be stored in the laboratory so that they 

 will not lose their magnetism so rapidly? 



6. Will two horseshoe magnets of equal strength cling to each 

 other? Explain. 



7. Why does soft iron lose its magnetism so quickly? 



Electrification by Friction. If a stick of sealing wax 

 or a piece of hard rubber is rubbed with a woolen cloth 

 or a piece of cat's fur, it will attract small pieces of paper, 

 pith balls, cork, and other light bodies. A glass rod 



